Add global bind plugin support

Add global bind plugin support.

**Example:**

```js
$scope.myKeybindings = {
  bindGlobal: {
    'ctrl+s': $scope.saveAction,
    'command+s': $scope.saveAction
  }
  'ctrl+b': $scope.backAction
};
```

Remove unused method
Merge pull request #8 from jrschumacher/patch-1

Add global bind plugin support